For the use of measuring assemblies including a numerical display of data it is of interest to have available a control system which permits verifying that the indications actually displayed are indeed conformable to the commands which have been sent to the displays. In case of non-concordance between the command given and the value displayed, an alarm can be given and possibly the measuring process can be stopped.
Such a system is required, for example, by the French regulations on electronic calculators used in fuel dispensers; the indications of unit price, volume delivered, amount to be paid are realized with the aid of numerical displays of seven segments.
U.S. Pat. No. 3,943,500 describes a device for the release of an alarm in case of defective functioning of a 7-segment display for the data of quantity and price of a fuel dispensing station. In this device, workable signals of intensity can be derived only with displays which produce their own light, that is, displays with incandescent filaments or with light-emitting diodes. In the case of liquid crystal displays this method cannot be applied because the current intensities are extremely low and hence practically impossible to check.
It is an object of the present invention to propose a control device for liquid crystal displays operating in multiplex, simple and cost-effective to realize, involving only numerical data.